Synoptic Controls on Entrainment Mixing, Shear, and The Three-Layer Atmosphere Above the San Joaquin Valley of California

Atmospheric Boundary Layer (ABL) dynamics, including enthalpy budgets and entrainment rates are presented from in-situ flight data taken on six afternoon flights around Fresno, CA in July/August 2016. The flights were conducted during the California Baseline Ozone Transport Study (CABOTS) to study the transport of air pollutants across and within the vast San Joaquin Valley. Entrainment velocity estimates were made based on an observational budget of the inversion height, and ranged from 1.7 – 5.1 cm/s, averaging 3.2 cm/s. This method depends directly on the WRF-derived subsidence rates, which averaged -1.5 cm/s in the valley, implying that the midday growth of the ABL (averaging 1.3 cm/s) only accounted for half of the entrainment velocity. A strong positive relationship was found between the normalized entrainment rates, w_e/w_*, and an inverse bulk Richardson number (r²=0.59), suggesting that vertical wind shear may play a significant role in driving entrainment at the top of the convective boundary layer in the valley. That is, the entrainment efficiency (ratio of entrainment to surface heat fluxes ranging from 0.08 – 0.41) is directly related to the wind speed at the ABL top. Additionally, a method is outlined to estimate turbulent kinetic energy (TKE) in the ABL from a 1 Hz airborne wind system using convective scale similarity, and the results are shown to correspond well to an independent method of estimating surface heat fluxes using an enthalpy scalar budget. We also present a three-layer conceptual model of the lower atmosphere over the valley: a convective boundary layer that only extends up to about 700 m in the afternoon due to strong mesoscale subsidence, a free troposphere above ~ 2000 m, and an intermediate 'buffer layer' that comprises a mix of the two. The buffer layer is characterized by stagnating horizontal winds due to the low Froude number, with boundary layer air mixed into it due to venting up the valley walls, and a dilution time scale of approximately a week.